1. Field of the Invention
This invention relates to a process for the continuous casting of a metal ingot having a smooth and beautiful surface.
2. Description of the Prior Art
A metal ingot obtained by continuous casting usually does not have a completely smooth surface, but presents an uneven, and often locally cracked surface. This is due to the use of a cold mold in any conventional continuous casting process. A solid skin defining the surface of an ingot is formed within the mold, and a friction develops between the skin of the ingot and the inner surface of the mold when the ingot moves through the mold. If an ingot having any such surface defect is subjected directly to working by plastic deformation, such as forging or rolling, there results a product having a number of defects. Therefore, surface scalping or scarfing of the ingot is required beforehand. If the ingot has too deep a crack, it cannot be subjected to such working, but must be remelted to form a satisfactory ingot.
According to the conventional continuous casting process employing a cold mold, the ingot usually leaves the mold through its bottom. If a solid skin formed by the metal to be cast adheres to the inner surface of the mold, the solid skin is prevented from moving toward the exit of the mold, and results in breakage. If such breakage occurs in the vicinity of the mold exit, the molten metal surrounded by the solid skin blows out through the bottom of the mold. This phenomenon is called a breakout, and not only disables the continuation of the casting operation, but also presents a serious threat to the safety of the operation. The breakout is particularly likely to occur to a metal or alloy having a wide solidification temperature range. Therefore, in order to prepare a continuous cast ingot of any such metal, for example, cast iron or phosphor bronze, there is no alternative but to rely on an intermittent process in which the molten metal is completely solidified with the mold. This process is very troublesome and time-consuming.
In order to solve this problem, the inventor of this invention has proposed a new process as disclosed in Japanese Patent Publication No. 46265/1980 published on Nov. 21, 1980. This process has, however, been still unsatisfactory in failing to provide a smooth and beautiful cast surface, though it has turned out to provide a more or less satisfactory solution to the problem of breakout. There has, therefore, been a demand for a further improvement.